1. Field of the Invention
The present invention relates to mobile radio communication systems and, more particularly, to a method and system for minimizing interference produced by mobile radio stations initiating communications with fixed radio stations.
2. Description of the Related Art
In cellular radio telecommunications networks, a mobile radio subscriber station can initiate a telephone call to a base station at any time through a process known as random access call set-up. The random access procedure applies not only to initiation of a call by the mobile station but also the mobile station's first transmission of a reply to a paging signal from the mobile station's currently associated base station. In each of these situations, the amount of power with which the mobile station transmits an initial access message to the base station can have a strong effect upon the overall quality of the communication between the base station and other mobile stations.
Two main techniques of modulation have been used in cellular radio telecommunication systems: analog and digital. Among the former, frequency division multiple access (FDMA) has historically been the most common way in which multiple call connections are handled between many mobiles and a single base station. In FDMA, the available radio frequency spectrum is divided into a plurality of channels. A separate and distinct frequency channel is assigned to each mobile station and that channel is occupied continuously throughout the length of its connection to that base station. More recently, digital modulation techniques have been introduced to increase the efficiency of spectrum utilization in cellular systems. The principal digital modulation methods employed are time division multiple access (TDMA) and code division multiple access (CDMA). In the TDMA technique different ones of the mobile stations are allocated a separate short time slot in a periodically repeating frame of information during which they transmit bursts of digitized data containing both speech and control information. Thus, several separate mobile stations time-share a single frequency channel previously occupied by a single mobile station and thereby increase the efficiency with which the available radio frequency spectrum is used. In the CDMA technique the speech and information signals from a plurality of mobile stations are each separately encoded with a different spread-spectrum pseudo-noise (PN) chip code that distinguishes its signal from that of the other mobiles. All of the PN encoded signals are then transmitted through spread-spectrum transmission over the same relatively broad frequency band. While all of the PN encoded signals overlap each other in both time and frequency they are decoded by correlation with the PN code associated with the desired speech/information signal of a particular mobile station.
Because mobile stations move freely within each of the cells of a cellular telecommunications system, the physical distance between each one of the mobile stations and its associated base station can vary significantly. Since the radio signal propagation loss between a transmitter and a receiver increases typically as a function of the fourth power of the distance between them, very large differences usually arise in the strength of the signals received at the base station from different ones of the mobile stations. There are numerous techniques for reducing the interference between the different signals of the respective mobile stations. Nevertheless interference is present as a result of the great disparity in distances between each of the mobile stations and their common base station and, thus, the great differences between the signal strengths of the various mobiles.
The power regulation problem associated with mobile stations is greatly exasperated in the case of CDMA systems where a mobile station receiving with twice the power of another mobile station occupies twice the system capacity of that station (assuming the mobile stations have the same perceived path loss). In addition, a mobile station transmitting at a higher data rate than another mobile station also occupies a greater portion of the system resources than that mobile station. While regulation of the power with which mobile stations transmit in general is extremely important in CDMA systems, the regulation of power with which a mobile station seeks initial access to a base station is especially critical.
As illustrated in FIG. 1, a conventional technique used by a mobile station seeking access to a base station in a radio telecommunications system is that of gradually increasing the power level at which an access request message is sent to the base station until an acknowledgment message from the base station is received back at the mobile. That is, an initial access message is first transmitted at a preselected power level and then if no acknowledgment is received from the base station, the mobile station repeats the access request message a few milliseconds later at a slightly higher power level. It continues gradually increasing the power of its transmissions until an acknowledgment of receipt of the access message from the base station is received by the mobile. The power at which access is actually granted to the mobile station will be at some value greater than the actual power which was necessary to achieve communication. This degree of "overshoot" is a function of the rapidness with which the transmission power of the mobile station is increased i.e., the faster the power is increased the greater the overshoot.
If access is never granted by the base station, e.g., due to lack of system capacity or necessary hardware resources, a mobile will increase its power level to its maximum before it gives up. Thereafter it may recycle and start the process over again. If many mobile stations are futility seeking access to the system and each mobile is transmitting an access request message at increasingly higher power levels, this can result in serious degradation of system resources, especially in CDMA systems.
Certain techniques have been proposed for the initial determination of an appropriate power with which to initiate access by a mobile station to a base station. For example, in U.S. Pat. No. 5,278,992, the mobile station performs a calculation based upon monitored transmission power and received signal level of the base station. However, such a technique cannot prevent attempted accesses by a mobile station which would ultimately be unsuccessful. They still result in the addition of an increase in the level of interference within the system and a consequent degradation of system performance.
Thus, there is a need for a technique of inhibiting access attempts by a mobile station which will either be unsuccessful or which will result in an inordinate degree of interference and degradation of system performance.